Membrane electrode assembly for electrochemical cell

ABSTRACT

Disclosed is a membrane electrode assembly that includes a polymer electrolyte membrane, a first electrochemical reaction layer formed on one side of the polymer electrolyte membrane to allow an oxidation reaction to occur thereon, a first electron-conductive layer formed between the polymer electrolyte membrane and the first electrochemical reaction layer, a second electrochemical reaction layer formed on a remaining side of the polymer electrolyte membrane to allow a reduction reaction to occur thereon, and a second electron-conductive layer formed between the polymer electrolyte membrane and the second electrochemical reaction layer. The first electron-conductive layer and the second electron-conductive layer include a porous metal.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a membrane electrode assembly for anelectrochemical cell, and more particularly to a membrane electrodeassembly for an electrochemical cell including a separate electronconduction layer for an electron movement path between electrodecatalysts therein.

2. Description of the Related Art

Generally, an electrochemical cell is an energy converter that useselectric energy or generates electric energy, and is classified into anelectrolytic cell and a fuel cell. In order to put the electrochemicalcell to practical use, the fuel cell needs to have improved outputdensity (reduced electric energy consumption in the case of waterelectrolysis), improved durability, and a low price.

FIGS. 1 to 4 show a unit structure of a typical electrochemical cell, anelectrochemical stack structure, and a system structure.

FIG. 1 is a view showing the concept of a membrane electrode assembly100 which constitutes a portion of a typical electrolytic cell, whichelectrochemically decomposes water to generate hydrogen and oxygengases. FIG. 1 shows the thickness of the layer of each of the elementsat a lower part thereof.

The electrochemical cell for electrolysis, which electrolyzes water(H₂O) to generate oxygen gas (O₂) and hydrogen gas (H₂), includes afirst electrochemical reaction layer 104, a second electrochemicalreaction layer 108, a membrane 106, a first diffusion layer 102, and asecond diffusion layer 110. The first electrochemical reaction layer 104includes a first electrochemical catalyst 112 and a first carrier 114,and the second electrochemical reaction layer 108 includes a secondelectrochemical catalyst 116 and a second carrier 118.

The first diffusion layer 102 and the second diffusion layer 110 help tomove electrons, reactants, or products to or from the first and thesecond electrochemical catalysts 112 and 116. The first and the secondelectrochemical catalysts 112 and 116 are the most important materialsthat are used to perform electrolysis or generate electric energy, andthe first and the second carriers 114 and 118 function to support thefirst and the second electrochemical catalysts 112 and 116 and providean electron movement path.

The first and the second electrochemical catalysts 112 and 116 are mixedwith the first and the second carriers 114 and 118, a binder, and asolvent to form a slurry or paste, which is then applied on the membrane106 or on the first and the second diffusion layers 102 and 110 to formthe first and the second electrochemical reaction layers 104 and 108.The manufactured assembly of “electrochemical reaction layers 104 and108-membrane 106” or “electrochemical reaction layers 104 and108-membrane 106-diffusion layers 102 and 110” is called a membraneelectrode assembly (hereinafter, referred to as “MEA”).

The interval between the first electrochemical reaction layer 104 andthe second electrochemical reaction layer 108, which are formed in theMEA, has a physical thickness value of the membrane. Bubbles are notpresent in the first electrochemical reaction layer 104 or in the secondelectrochemical reaction layer 108, thereby making it possible toperform high-current operation at low voltages. Further, since theconductivity of an electrolyte solution is not used, unlike in an alkalielectrolytic cell, water, which is a raw material, may be used whileensuring high purity, and accordingly, high-purity hydrogen and oxygenmay be obtained.

The process of electrolyzing water will be described below using theconstitution shown in FIG. 1. The place at which an oxidation reactionoccurs is considered the first electrochemical reaction layer 104, andthe place at which a reduction reaction occurs is considered the secondelectrochemical reaction layer 108. The oxidation and reductionreactions occur simultaneously.

First, when water (H₂O) is supplied through the first diffusion layer102 to the first electrochemical reaction layer 104, the water isdecomposed into oxygen gas (O₂), electrons (e⁻) , and hydrogen ions (H⁺)(protons) at the first electrochemical catalyst 112 (also called anoxidation catalyst, an anode active material, or an oxygen-gasgenerating electrode), as shown in the following Reaction Scheme 1. Theoxygen gas (O₂) is discharged to the outside of the electrolytic cellvia diffusion, and the hydrogen ions (H⁺) are moved through the membrane106 to the second electrochemical catalyst 116 (also called a reductioncatalyst, a cathode active material, or a hydrogen-gas generatingelectrode) by an electric field. The electrons (e⁻), which are generateddue to the aforementioned reaction, are moved from the firstelectrochemical catalyst 112 through the first diffusion layer 102 andan external circuit (not shown) to the second diffusion layer 110 andthe second electrochemical catalyst 116.

Meanwhile, the hydrogen ions (H⁺) and the electrons (e⁻), which aremoved from the first electrochemical catalyst 112, react at the secondelectrochemical catalyst 116 to generate hydrogen gas (H₂), as shown inReaction Scheme 2. In addition, a portion of the water supplied to thefirst electrochemical reaction layer 104 is moved to the secondelectrochemical reaction layer 108 by an electric field to thus bedischarged together with the hydrogen gas (H₂) to the outside of theelectrolytic cell.

The electrochemical reactions, which occur at the first electrochemicalcatalyst 112 and the second electrochemical catalyst 116, are shown inthe following Reaction Schemes 1 and 2. The overall reaction at thefirst electrochemical catalyst 112 and the second electrochemicalcatalyst 116 is shown in Reaction Scheme 3.

2H₂O→4H⁺+4e ⁻+O₂ (Anode)   [Reaction Scheme 1]

4H⁺+4e ⁻→2H₂ (Cathode)   [Reaction Scheme 2]

2H₂O→O₂+2H₂   [Reaction Scheme 3]

Meanwhile, a reverse reaction of electrolysis of water occurs in thefuel cell, and will be described below (See Reaction Schemes 4 to 6).

First, hydrogen gas is introduced into a first electrochemical reactionlayer 104, and oxygen gas is supplied to a second electrochemicalreaction layer 108. The hydrogen gas is then converted into hydrogenions (proton) and electrons via an electrochemical reaction at a firstelectrochemical catalyst 112, and the electrons are moved through anexternal load, which is electrically connected to the fuel cell, and theprotons are moved through a membrane to a second electrochemicalcatalyst 116. The protons and the electrons, which are generated at andmoved from the first electrochemical catalyst 112, react with oxygengas, which is supplied from the outside, at the second electrochemicalcatalyst 116 to generate water, energy, and heat.

2H₂→4H⁺+4e ⁺(Anode)   [Reaction Scheme 4]

4H⁺+O₂+4e ⁻→2H₂O (Cathode)   [Reaction Scheme 5]

O₂+2H₂→2H₂O   [Reaction Scheme 6]

The following description pertains mainly to water electrolysis for theelectrolysis of water, but is applicable not only to water electrolysisbut also to fuel cells.

FIG. 2 is a view showing the structure of a typical electrochemical cellwhich includes the MEA of FIG. 1 to electrolyze water. Anelectrochemical cell 200, like that shown in FIG. 2, includes a firstend plate 202, a first insulating plate 204, a first current applicationplate 206, a first electrochemical reaction chamber frame 208, a firstelectrochemical reaction chamber 210, the MEA 100 of FIG. 1, a secondelectrochemical reaction chamber 212, a second electrochemical reactionchamber frame 214, a second current application plate 216, a secondinsulating plate 218, and a second end plate 220. A direct current powersupply is used as a power converter 224, which applies current to theelectrochemical cell.

The first end plate 202 and the second end plate 220 have bolt/nutfastening holes (not shown) for assembling the unit electrochemicalcells, and provide paths (not shown) through which reactants andproducts are moved. The first insulating plate 204 and the secondinsulating plate 218 provide an electric insulation function between thefirst end plate 202 and the first current application plate 206 andbetween the second end plate 220 and the second current applicationplate 216. The first current application plate 206 and the secondcurrent application plate 216 are connected to the power converter 224to apply required current to the electrochemical cell 200.

Meanwhile, when the first electrochemical catalyst 112 is positioned inthe first electrochemical reaction chamber 210 to allow an oxidationreaction to occur, the first electrochemical reaction chamber 210becomes a space through which water, as the reactant, and oxygen, as theproduct, are moved. The second electrochemical reaction chamber 212,which is positioned at the opposite side of the first electrochemicalreaction chamber 210 while the membrane 106 is interposed between thefirst and the second electrochemical reaction chambers, provides a spacethrough which hydrogen, which is generated in the reduction reaction,and water, which is moved from the first electrochemical reactionchamber 210, are moved.

The first electrochemical reaction chamber 210 is isolated from theoutside by the first electrochemical reaction chamber frame 208, and thesecond electrochemical reaction chamber 212 is isolated from the outsideby the second electrochemical reaction chamber frame 214. In addition, agasket (or packing) 222 is provided between the MEA 100 and the firstelectrochemical reaction chamber frame 208 and between the MEA 100 andthe second electrochemical reaction chamber frame 214 in order toprevent the reactants and the products from leaking to the outside.

Among the elements constituting the electrochemical cell 200, the firstelectrochemical reaction chamber frame 208, the second electrochemicalreaction chamber frame 214, and the gasket 222 have predetermined holes,through which the reactants or the products are easily supplied to anddischarged from the electrochemical cell. The first electrochemicalreaction chamber frame 208 and the second electrochemical reactionchamber frame 214 have fluid paths (represented by the dotted line in(A) of FIG. 2).

Meanwhile, another electrochemical cell 200 may have a pressure pad (notshown, refer to 304 of FIG. 3) between the second electrochemicalreaction chamber frame 214 and the second current application plate 216so as to maintain the balance of the electrochemical cell 200.

FIG. 3 is a view showing the concept of a known electrochemical stack. Aplurality of unit electrochemical cells is required in order to obtain adesired amount of products during an electrolysis reaction, and anassembly of the two or more layered electrochemical cells is called anelectrochemical stack.

When the electrochemical cells are layered in order to constitute theelectrochemical stack 300 shown in FIG. 3, unit electrochemical cellsare repeatedly disposed in a desired number between the basicelectrochemical cells 200. A pressure pad 304 is interposed between theunit electrochemical cells in order to press the elements to each other.Bolts 306 are fastened with nuts 310 through holes, which are formedthrough edges of the first and the second end plates 202 and 220, inorder to assemble the unit electrochemical cells in the electrochemicalstack.

FIG. 4 is a view showing a system for electrolyzing water using theelectrolysis stack that is the same as the electrochemical stack of FIG.3 in order to produce hydrogen. A hydrogen-generating system 400, asshown in FIG. 4, includes an electrolysis stack 420, a water-treatingunit for treating water, which is supplied to the electrolysis stack420, and a gas-treating unit for purifying hydrogen gas, which isgenerated from the electrolysis stack 420, and controlling pressure.

Pure water of 1 Mega ohm cm or more is used as water, which is a rawmaterial used in the electrolysis stack 420. An automatic valve 402,which is provided in a pure water-supplying line s1, is adjusted tosupply pure water, and the automatic valve 402 is controlled using alevel sensor 405, which is used to sense the level, in an oxygen-waterseparation bath 404 (dotted line e2). Water is supplied from theoxygen-water separation bath 404 to the electrolysis stack 420 using acirculation pump 406, which is provided in a circulation pipe s2, joinswater circulating through a circulation line s9 from a hydrogen-waterseparation bath 424, and then passes through a pipe in which a heatexchanger 408, a water-quality sensor 410, and an ion-exchanging filter412 are provided. The water is then supplied to a first electrochemicalreaction chamber 414 (the place in which an oxidation reaction occurs)of the electrolysis stack 420. Meanwhile, when direct current issupplied from a power converter 440 through a wire e1 to theelectrolysis stack 420, the water undergoes a decomposition reaction.

Oxygen, which is generated from the first electrochemical reactionchamber 414, and unreacted water are moved through a discharge pipe s4to the oxygen-water separation bath 404, and a temperature sensor 416 isprovided in the discharge pipe s4 to sense the temperature. Oxygen,which is separated in the oxygen-water separation bath 404, isdischarged through an oxygen-discharge pipe s5 to the outside, and thewater is subjected to a re-circulation process.

The hydrogen gas, which is generated from the second electrochemicalreaction chamber 422, entails water, and is moved through a dischargepipe s6 to the hydrogen-water separation bath 424 so as to be separatedfrom water. A level sensor 426, which is used to sense the level, isprovided in the hydrogen-water separation bath 424 so as to adjust thelevel. When the level of the hydrogen-water separation bath 424 is apredetermined value or more, an automatic valve 428 is opened (electricsignal of e3) to supply the water through the circulation line s9 to thecirculation pipe s2.

Meanwhile, the hydrogen gas, which is separated in the hydrogen-waterseparation bath 424, is supplied through a gas pipe s7 to a hydrogen-gaspurifier 430 to thus remove moisture from hydrogen. Typically, a bed,which is filled with a moisture absorbent, is applied to the hydrogengas purifier 430. The hydrogen that passes through the hydrogen gaspurifier 430 is supplied through a high-purity hydrogen gas pipe s8 to afield requiring hydrogen. A pressure control valve 434 is provided inthe high-purity hydrogen gas pipe s8 to control the pressure of thehydrogen gas generated from the electrolysis stack 420. Pressure sensors432 and 438 are provided in the front and the rear of the pressurecontrol valve 434 to measure pressure, and a check valve 436 is providedto maintain the flow of gas in a predetermined direction.

In this water electrolysis system, a reduction in electric energyconsumption (in the case of a fuel cell, an increase in output density),improvement in durability, and cost reduction are required in order torealize practical use of an electrochemical cell. In this regard, in thecase of the structure of the conventional MEA 100, since the structurefor delivering electrons between the electrode catalysts in the MEA isnot developed, the electrochemical activity is low and it is difficultto realize performance at high current density.

FIG. 5 shows photographs of the catalyst distribution on the surface ofa typical MEA (Comparative Example 1, to be described later)manufactured using a conventional method. As seen from FIG. 5, theelectrochemical reaction layer is locally formed, causing a lack ofelectron delivery layers (black portions in the photographs).Accordingly, there is a problem in that a large amount of catalyst isconsumed in order to maintain the electrochemical performance.

CITATION LIST Patent Document

Korean Patent No. 10-1357146

Korean Patent Application Publication No. 10-2008-0032962

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made keeping in mind theabove problems occurring in the related art, and an object of thepresent invention is to provide a membrane electrode assembly (MEA) foran electrochemical cell including a separate electron conduction layerfor an electron movement path between electrode catalysts therein. Inthe MEA, it is possible to enable high current density operation, reduceelectric energy consumption even using typical current density, improvedurability, and reduce manufacturing costs.

In order to accomplish the above object, the present invention providesa membrane electrode assembly for an electrochemical cell that includesa polymer electrolyte membrane, a first electrochemical reaction layerformed on one side of the polymer electrolyte membrane to allow anoxidation reaction to occur thereon, a first electron-conductive layerformed between the polymer electrolyte membrane and the firstelectrochemical reaction layer, a second electrochemical reaction layerformed on a remaining side of the polymer electrolyte membrane to allowa reduction reaction to occur thereon, and a second electron-conductivelayer formed between the polymer electrolyte membrane and the secondelectrochemical reaction layer. The first electron-conductive layer andthe second electron-conductive layer include a porous metal.

Further, according to the present invention, the porous metal mayfurther include a coating layer including a platinum group applied on asurface thereof.

Further, according to the present invention, the firstelectron-conductive layer and the second electron-conductive layer mayeach have a thickness of 0.1 to 1 mm.

Further, according to the present invention, the firstelectron-conductive layer and the second electron-conductive layer mayinclude any one among platinum, palladium, rhodium, iridium, ruthenium,osmium, carbon, gold, tantalum, tin, indium, nickel, tungsten,manganese, and titanium, or a complex including two or more thereof.

Further, according to the present invention, the first electrochemicalreaction layer and the second electrochemical reaction layer may eachinclude an electrochemical catalyst (or the electrochemical catalyst ona carrier), an ionic conductor, and a binder.

Further, according to the present invention, the electrochemicalcatalyst may include any one among platinum group elements includingplatinum, palladium, ruthenium, iridium, rhodium, and osmium, any onemetal among iron, lead, copper, chromium, cobalt, nickel, manganese,vanadium, molybdenum, gallium, and aluminum, or alloys, oxides, ordouble oxides thereof.

Further, according to the present invention, elements constituting themembrane electrode assembly may include the polymer electrolytemembrane, the first electron-conductive layer, the secondelectron-conductive layer, the first electrochemical reaction layer, andthe second electrochemical reaction layer in descending order of size.

According to the present invention, for an electron movement path,electrons are sequentially moved through an anode catalyst, anelectron-conductive layer on a membrane, an external circuit, theelectron-conductive layer on the membrane, and a cathode catalyst.Therefore, the electron movement path is short compared to a knownelectrochemical cell having an electron movement path, which is formedso that the electrons are sequentially moved through an anode catalyst,an anode-chamber diffusion layer, an anode-chamber current applicationplate, an external circuit, a pressure pad, a cathode-chamber currentapplication plate, a cathode-chamber diffusion layer, and a cathodecatalyst. Accordingly, the current density-voltage characteristics ofthe electrochemical cell are excellent, thereby reducing energyconsumption during electrolysis.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more clearly understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a view showing the concept of an MEA, which is a portion of atypical electrolytic cell that electrochemically decomposes water toproduce hydrogen and oxygen gases;

FIG. 2 is a view showing the structure of a typical electrochemical cellwhich includes the MEA of FIG. 1 to electrolyze water;

FIG. 3 is a view showing the concept of a known electrochemical stack;

FIG. 4 is a view showing a system for electrolyzing water using theelectrochemical stack of FIG. 3 to produce hydrogen;

FIG. 5 shows photographs of the catalyst distribution on the surface ofa typical MEA manufactured using a conventional method;

FIG. 6 is a view showing an MEA according to an embodiment of thepresent invention; and

FIG. 7 is a comparative graph showing the performance of Example 1 ofthe present invention and Comparative Example 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the appended drawings so as toeasily perform the present invention by the person having ordinary skillin the related art. However, descriptions of known techniques, even ifthey are pertinent to the present invention, are considered unnecessaryand may be omitted insofar as they would make the characteristics of theinvention unclear. Furthermore, the same or similar portions arerepresented using the same reference numeral in the drawings.

FIG. 6 is a view showing an MEA according to an embodiment of thepresent invention. As shown in FIG. 6, an MEA 500 according to theembodiment of the present invention includes a first electron-conductivelayer 506, a first electrochemical reaction layer 504, a membrane 502, asecond electron-conductive layer 518, and a second electrochemicalreaction layer 508. The first electron-conductive layer 506 and thefirst electrochemical reaction layer 504 are sequentially formed on oneside of the membrane 502, and the second electron-conductive layer 518and the second electrochemical reaction layer 508 are sequentiallyformed on the remaining side of the membrane 502. Meanwhile, the MEA 500may or may not include a diffusion layer.

An electrolysis reaction of water in the MEA 500 of the presentembodiment will be described below. The description will be given on theassumption that an oxidation reaction (oxygen generation reaction)occurs at a first electrochemical catalyst and a reduction reaction(hydrogen generation reaction) occurs at a second electrochemicalcatalyst.

First, when water (H₂O) is supplied to a first electrochemical catalyst510 (oxidation catalyst, oxygen catalyst), water is decomposed intooxygen gas (O₂), electrons (e⁻), and hydrogen ions (H⁺) (protons). Aportion of the water (H₂O) is discharged to the outside together withthe oxygen gas (O₂), and the hydrogen ions (H⁺), which are obtained dueto decomposition, are moved through the membrane 502 to a secondelectrochemical catalyst 516 (reduction electrode, hydrogen electrode).

In addition, the decomposed electrons are moved to an external circuit(not shown) via the first electron-conductive layer 506 formed on themembrane 502 and the first electrochemical catalyst 510 (oxidationcatalyst, oxygen catalyst). Meanwhile, the electrons (e⁻) moved alongthe external circuit (not shown) for connecting the firstelectron-conductive layers 506 are moved through the secondelectron-conductive layer 518. The electrons moved to the secondelectron-conductive layer 518 reach the first electrochemical catalyst510. The electrons that are moved react with hydrogen ions (H⁺) in thesecond electrochemical reaction layer 508 to generate hydrogen gas. Inaddition, water (H₂O) which has passed through the membrane 502 inconjunction with the hydrogen ions (H⁺) is discharged to the outside ofthe electrolytic cell together with the hydrogen gas. Theelectrochemical reaction that occurs under the first and the secondelectrochemical catalysts 510 and 516 is shown in the aforementionedReaction Schemes 1 and 2.

Any membrane may be used as the membrane 502 of the present embodimentas long as the membrane has hydrogen ion (proton) conductivity, and afluorine-based polymer electrolyte and a hydrocarbon-based polymerelectrolyte may be used. Examples of the fluorine-based polymer membranemay include Nafion (Registered trademark), manufactured by the DuPontCompany, Flemion (Registered trademark), manufactured by Asahi GlassCo., Ltd., Aciplex (Registered trademark) manufactured by Asahi KaseiCorporation, and Gore Select (Registered trademark) manufactured by Gore& Associates, Inc. Examples of the hydrocarbon-based polymer membranemay include an electrolyte membrane such as sulfonated polyether ketone,sulfonated polyether sulfone, sulfonated polyether ether sulfone,sulfonated polysulfide, and sulfonated polyphenylene. Among theaforementioned examples, it is preferable to use a Nafion (Registeredtrademark)-based material, which is manufactured by the DuPont Company,as the polymer membrane.

The first and the second electron-conductive layers 506 and 518 of thepresent embodiment are formed on either side of the membrane 502, andfunction to conduct electrons. The first and the secondelectron-conductive layers 506 and 518 formed on the membrane 502 are0.1 to 1 mm and preferably 0.1 to 0.5 mm in thickness. This is becausewhen the thickness of the first and second electron-conductive layers506 and 518 is 0.1 mm or less, it is difficult to form theelectrochemical reaction layer on the electron-conductive layer, andwhen the thickness of the electron-conductive layer is 1 mm or more, theexcessive formation of the electrochemical reaction layer interfereswith the movement of the proton, thereby lowering the ionicconductivity. The material of the first and second electron-conductivelayers 506 and 518 may be a metal having excellent conductivity such asplatinum, palladium, rhodium, iridium, ruthenium, osmium, carbon, gold,tantalum, tin, indium, nickel, tungsten, manganese, and titanium. It ispreferable that a platinum group coating be applied on titanium from theviewpoint of chemical resistance.

Meanwhile, the first and second electron-conductive layers 506 and 518may include a metal or may include a coating layer including a platinumgroup applied on the metal. Meanwhile, a material having excellentconductivity may be formed on the metal by the principle of electrolessplating of a process of precipitating a metal precursor with a reducingagent, or by thermal decomposition of the metal precursor.

The first and the second electrochemical reaction layers 504 and 508 ofthe present embodiment are formed on either side of the membrane 502having the first and the second electron-conductive layers 506 and 518.The first and second electrochemical reaction layers 504 and 508 mayinclude an electrochemical catalyst (or the electrochemical catalyst ona carrier), an ionic conductor, and a binder. The catalyst ink for thefirst electrochemical reaction layer 504 includes at least a firstelectrochemical catalyst 510, a carrier 512 (may not be included), apolymer electrolyte, and a solvent, and the catalyst ink for the secondelectrochemical reaction layer 508 includes at least a secondelectrochemical catalyst 516, a carrier 514 (may not be included), apolymer electrolyte, and a solvent.

Examples of the polymer electrolyte included in the catalyst ink of thepresent embodiment may include a fluorine-based polymer electrolyte anda hydrocarbon-based polymer electrolyte exhibiting proton conductivity.In addition, examples of the fluorine-based polymer electrolyte mayinclude a Nafion (Registered trademark)-based material manufactured bythe DuPont Company. Examples of the hydrocarbon-based polymerelectrolyte may include an electrolyte such as sulfonated polyetherketone, sulfonated polyether sulfone, sulfonated polyether ethersulfone, sulfonated polysulfide, and sulfonated polyphenylene.Considering the adhesion between the first and second electrochemicalreaction layers 504 and 508 and the first and second electron conductivelayers 506 and 518, it is preferable to use the same material as themembrane 502, that is, the Nafion ionomer.

Examples of the first and second electrochemical catalysts 510 and 516used in the first electrochemical reaction layer 504 and the secondelectrochemical reaction layer 508 in the present embodiment may includeplatinum group elements including platinum, palladium, ruthenium,iridium, rhodium, and osmium, metal such as iron, lead, copper,chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, andaluminum, or alloys, oxides, or double oxides thereof. It is preferableto use one or more metals, which are selected from platinum, palladium,rhodium, ruthenium, and iridium, or oxides thereof in order to ensureexcellent electrode reactivity and effectively perform a stableelectrode reaction over a long period of time.

In the present embodiment, when the particle diameter of the first andthe second electrochemical catalysts 510 and 516 is very large, theactivity of the catalyst is reduced, and when the particle diameter isvery small, the stability of the catalyst is reduced. Accordingly, theparticle diameter is preferably 0.5 to 20 nm, and more preferably 1 to 5nm.

Meanwhile, carriers 512 and 514 for carrying the catalyst include powderthat exhibits electron conductivity, and titanium oxide or carbonparticles may be used. The carrier is provided in a fine particle formand exhibits conductivity, and any carrier may be used as long as thecarrier does not intrude the catalyst. However, it is preferable to usetitanium oxides, carbon black, graphites, black lead, activated carbon,carbon fibers, carbon nanotubes, or fullerene.

In addition, when the particle diameter of the carriers 512 and 514 isvery small, it is difficult to form an electron-conductive path, andwhen the particle diameter is very large, the diffusion of gas into theelectrode catalyst layer formed on the carrier is reduced, or theavailability of the catalyst is reduced. Accordingly, the particlediameter is preferably 10 to 1,000 nm, and more preferably 10 to 100 nm.

In the present embodiment, the thickness of the first and secondelectrochemical reaction layers 504 and 508 is the same as or largerthan that of the first and second electron-conductive layers 506 and518, and is preferably 0.1 mm or less.

Meanwhile, the size of each component constituting the MEA 500 of thepresent embodiment is as follows. The size Dc of the membrane 502 islarger than that of first and second electrochemical reaction chamberframes 208 and 214 in FIG. 2, and the size Db of the first and secondelectron-conductive layers 506 and 518 is smaller than that of the firstand second electrochemical reaction chamber frames 208 and 214 in FIG.2. The size Da of the first and second electrochemical reaction layers504 and 508 is preferably the same as the internal area of first andsecond electrochemical reaction chambers 210 and 212 in FIG. 2. Further,the components constituting the MEA 500 preferably include the membrane502, the first electron-conductive layer 506, the secondelectron-conductive layer 518, the first electrochemical reaction layer504, and the second electrochemical reaction layer 508 in descendingorder of size.

A method of manufacturing an MEA according to the embodiment of thepresent invention will be described hereinafter.

Process 1: Pre-Treatment Process of the Membrane 502

Pre-treatment of the membrane 502 is a process which includes rougheningthe surface of the membrane using a mechanical process, and physicallyand chemically treating organic and inorganic impurities present in themembrane 502. The process will be described in detail below.

Process 2: Process of Forming the First and the SecondElectron-Conductive Layers 506 and 518

The first and second electron-conductive layers 506 and 518 areintegrated with the membrane 502 obtained during process 1. A metalprecursor solution having electron conductivity (for example, copper,platinum, silver, gold, etc.) and a reducing agent are added to a porousmetal to form a metal thin film layer having electron conductivity onthe porous metal, thereby forming the first and secondelectron-conductive layers 506 and 518. The thickness of the metal thinfilm layer is obtained by repeating a deposition reduction process. Thedetailed procedure thereof will be described later. The procedure isconstituted based on the principle of electroless plating, but theconstitution is not limited thereto. The metal precursor may bethermally decomposed, thus forming the metal thin film layer on theporous metal. Meanwhile, the first and second electron-conductive layers506 and 518 that are manufactured in the above-described manner may besimply integrated with the membrane 502 by thermal pressing using a hotpress.

Process 3: Process of Forming the First and the Second ElectrochemicalReaction Layers 504 and 508

Process 3 is a process of forming the first and the secondelectrochemical reaction layers 504 and 508 on the membrane 502 whichhas the first and the second electron-conductive layers 506 and 518obtained during process 2. Process 3 includes a catalyst synthesisprocess, a catalyst ink manufacturing process, a catalyst ink transferprocess, and a thermal-pressing process. The catalyst synthesis processincludes forming mixture oxides, using a reaction of a desired catalystprecursor and an oxidant, and drying the mixture oxides to obtain anelectrochemical catalyst having a powder structure. The catalyst inkmanufacturing process includes mixing the electrochemical catalyst,which is synthesized during the catalyst synthesis process, aparticulate material, a dispersant, and a binder made of the samematerial as the membrane 502 to manufacture the catalyst ink. Further,the catalyst ink transfer process includes transferring the catalystink, which is manufactured during the catalyst ink manufacturingprocess, onto a Teflon sheet using a spray and then drying the catalystink. The thermal-pressing process includes attaching the Teflon sheet,which is obtained during the catalyst ink transfer process, to bothsides of the membrane 502 having the first and the secondelectron-conductive layers 506 and 518, which are formed during process2, and then thermal-pressing the Teflon sheet using a hot press. Theprocesses will be described in detail below.

Hereinafter, the method of manufacturing the membrane electrode assemblyaccording to Examples of the aforementioned embodiment and ComparativeExamples will be specifically described, and experimental results willbe given. However, the present invention is not limited to the followingExamples.

EXAMPLE 1

1. Manufacture of the MEA 500

(1) Process 1: Pre-Treatment Process of the Membrane 502

Both surfaces of the membrane 502 (Nafion 117) were scratched in fourdirections using sandpaper (Emery sand paper 1100CW), and then subjectedto the swelling process in pure water at 90° C. Impurities were removedfrom the membrane, which was subjected to the swelling process, usingultrasonic wave treatment in pure water, and the membrane was treated in3% hydrogen peroxide (H₂O₂) for 30 min and in 0.5 to 1M sulfuric acid(H₂SO₄) at 90° C. for 30 min, and then subjected to the aforementionedpure water process again.

(2) Process 2: Process of Forming First and Second Electron-ConductiveLayers 506 and 518

A titanium porous metal (thickness 0.2 mm, porosity 60%) was depositedin a chloroplatinate ((NH₃)₄PtCl₂.H₂O) precursor solution for 5 hours.After the deposition process, a NaBH₄ solution was added dropwise every20 minutes for 2 hours in order to reduce the metal precursor. After thereduction was finished, platinum-plated titanium porous metal wasobtained. In order to obtain a plated layer having a desired thickness,the impregnation reduction process was repeated as many times asdesired, thus forming the first and second electron-conductive layers506 and 518. The first and second electron-conductive layers 506 and518, which were manufactured in the above-described manner, were thenintegrated with both sides of the membrane 502. The membrane 502 and thefirst and second electron-conductive layers 506 and 518 were integratedby being thermally pressed using a hot press.

(3) Process 3: Process of Forming First and Second ElectrochemicalReaction Layers 504 and 508

During the process of forming the first and the second electrochemicalreaction layers 504 and 508, the first and the second electrochemicalreaction layers 504 and 508 were formed on the membrane 502 having thefirst and the second electron-conductive layers 506 and 518. The processof forming the first and the second electrochemical reaction layers 504and 508 included a process of synthesizing first and secondelectrochemical catalysts, a process of manufacturing ink for the firstand the second electrochemical catalysts, a process of transferring inkfor the first and the second electrochemical catalysts, and athermal-pressing process.

(3-1) Process 3-1: Process of Synthesizing the First and the SecondElectrochemical Catalysts

(3-1-1) Synthesis of the First Electrochemical Catalyst

An oxidized iridium-ruthenium mixture catalyst was manufactured using areaction of iridium chlorides (IrCl₃.xH₂O) and ruthenium chlorides(RuCl₃.xH₂O) in a sodium nitrate solution. In addition, iridiumchlorides and ruthenium chlorides were agitated in the solution havingsodium nitrates dissolved therein for about 2 hours to be uniformlydissolved. The manufactured mixture catalyst solution was heated to 100°C. to vaporize distilled water for 1 hour to thus perform concentration,and the concentrate was sintered in an electric furnace at 475° C. for 1hour and then slowly cooled. Subsequently, the resulting material waswashed with 9 L of distilled water and filtered in order to removegenerated sodium chlorides. The obtained solid was dried at 80° C. for12 hours to manufacture a final iridium-ruthenium electrochemicalmixture catalyst.

(3-1-2) Synthesis of the Second Electrochemical Catalyst

Commercially available Pt/C (Premetek Inc., amount of carried platinumof 30%) was used as the second electrochemical catalyst 516.

(3-2) Process 3-2: Process of Manufacturing Ink for the First and theSecond Electrochemical Catalysts

(3-2-1) Manufacture of Ink for the First Electrochemical Catalyst

The oxidized iridium-ruthenium catalyst, which was manufactured duringprocess 3-1, nano-sized titanium dioxides as the carrier, and the Nafionsolution as the binder were used, and the used catalyst and Nafionionomers were mixed in an isopropyl alcohol solvent at a ratio of 1:3.5based on the weight of the solid. Agitation for 1 hour and ultrasonicwave treatment for 1 hour were alternately performed twice in order todisperse the catalyst.

(3-2-2) Manufacture of Ink for the Second Electrochemical Catalyst

Pt/C (Premetek Inc., amount of carried platinum of 30%) was used as thesecond electrochemical catalyst, and the Nafion solution (a registeredproduct from DuPont) was used as the binder. The used catalyst andNafion solution were mixed in an isopropyl alcohol solvent at a ratio of1:7.5 based on the weight of the solid. Agitation for 1 hour andultrasonic wave treatment for 1 hour were alternately performed twice inorder to disperse the catalyst.

(3-3) Process 3-3: Process of Transferring the First and the SecondElectrochemical Catalysts

(3-3-1) Transferring of the First Electrochemical Reaction Layer 504

The polytetrafluoroethylene (PTFE) sheet was used as the transfer sheet.The ink for the first electrochemical catalyst, which was obtainedduring process 3-2, was moved to a syringe for electrospraying only. Thecatalyst ink was transferred onto the base material and then dried inthe atmosphere at 90° C. for 30 min to manufacture an electrochemicalcatalyst layer. The amount of carried oxide catalyst was adjusted toabout 4 mg/cm² to set the thickness of the first electrochemicalreaction layer 504.

(3-3-2) Transferring of the Second Electrochemical Reaction Layer 518

The ink for the second electrochemical catalyst 516, which was obtainedduring process 3-2, was moved to a syringe for electrospraying only. Thecatalyst ink was transferred onto the carbon sheet and then dried in theatmosphere at 90° C. for 30 min to manufacture an electrochemicalcatalyst layer. The amount of the carried oxide catalyst was adjusted toabout 1 mg/cm² to set the thickness of the second electrochemicalreaction layer 508.

(3-4) Process 3-4: Thermal-Pressing Process

(3-4-1) Formation of the First Electrochemical Reaction Layer 504

The first electrochemical catalyst, which was obtained during process3-3 and loaded on the Teflon sheet, was thermal-pressed twice on themembrane 502, which was obtained during process 2, under a condition of120° C. and pressure of 10 MPa for 3 min. The Teflon sheet was removedto transfer the catalyst.

(3-4-2) Formation of the Second Electrochemical Reaction Layer 508

The carbon sheet, on which the manufactured second electrochemicalcatalyst 516 was loaded, was thermal-pressed under a condition of 120°C. and pressure of 10 MPa for 2 min on the opposite surface of themembrane 502, with which the manufactured first electrochemical reactionlayer 504 was combined, to obtain the MEA 500 shown in FIG. 6.

2. Electrochemical Cell for Evaluation and Evaluation System

The MEA of Example 1 had an electrochemically active area of 314 cm²(Da=20 cm), an electron-conductive layer thickness of 0.5 mm, Db of 21cm, an area of 346 cm², and a membrane size Dc of 25 cm. Titanium fiberswere layered on the first electrochemical reaction layer 504, and carbonfibers having high diffusibility were layered on the secondelectrochemical reaction layer 508 to perform evaluation. A cell forevaluation, shown in FIG. 2, and a water electrolysis system, shown inFIG. 4, were actually manufactured to perform evaluation.

The temperature of the cell was maintained at 80° C. (a temperaturesensor 416 in FIG. 4), and the current-voltage measurement of the cellfor evaluation was performed. Meanwhile, the discharge pressure ofhydrogen (controlled using s8 and 434 in FIG. 4) was maintained at about10 bar.

3. Measurement Result

From FIG. 7, it can be seen that the MEA manufactured in Example 1 has asmall voltage change, that is, a small slope, depending on currentdensity even when the current density is increased. FIG. 7 is acomparative graph showing the performance of Example 1 of the presentinvention and Comparative Example 1, and shows the performance of thecurrent density depending on voltage.

COMPARATIVE EXAMPLE 1

1. Manufacture of the MEA (Manufacture of the MEA According to a KnownMethod)

The pre-treatment process of the membrane and the processes of formingthe first and the second electrochemical reaction layers were performedusing the same procedure and conditions as Example 1, and the processesof forming the first and the second electron-conductive layers were notperformed in order to compare Comparative Example 1 and Example 1.

FIG. 5 shows photographs of the catalyst distribution on the surface ofa typical MEA that is manufactured using a conventional method and isused as Comparative Example 1, and also shows the concentrationdistribution for each noble metal. It can be seen that the catalyst onthe surface is not uniformly distributed, as indicated by the arrows inthe photographs.

2. Electrochemical Cell for Evaluation and Evaluation System

The same evaluation was performed in the electrochemical cell andevaluation system to which the MEA (electrochemically active area of 314cm²) of Comparative Example 1 was applied, like in Example 1.

3. Measurement Result

From FIG. 7, it can be seen that the voltage slope is significantlyincreased as the current density is increased in Comparative Example 1.

[Evaluation of Example 1 and Comparative Example 1]

FIG. 7 shows the current density-voltage characteristic of the MEAs ofExample 1 and Comparative Example 1, region (1) shows the high and lowperformance, depending on the electrochemical catalyst, and region (2)is a high current density region. The constitutions of theelectrochemical catalyst of Example 1 and the electrochemical catalystof Comparative Example 1 are the same. Accordingly, from FIG. 7, it canbe seen that the MEA of Example 1 and the MEA of Comparative Example 1have similar performance in region (1). However, it can be seen thatsince the membrane having the electron-conductive layer of Example 1 hasexcellent electron conductivity to the electrochemical reaction layer inthe MEA, the membrane exhibits the low-voltage characteristic in region(2), which is the high-density current region, compared to the membraneof Comparative Example 1.

As described above, the present invention has a better electron movementpath than the conventional electrochemical cell, and accordingly thecurrent density-voltage characteristic of the electrochemical cell isexcellent and the energy consumption required for electrolysis isreduced. In other words, in the related art, electrons are sequentiallymoved through an anode catalyst, an anode-chamber diffusion layer, ananode-chamber current application plate, an external circuit, a pressurepad, a cathode-chamber current application plate, a cathode-chamberdiffusion layer, and a cathode catalyst to thus form the electronmovement path. However, according to the present invention, theelectrons are sequentially moved through an anode catalyst, anelectron-conductive layer on a membrane, an external circuit, theelectron-conductive layer on the membrane, and a cathode catalyst tothus form the electron movement path. Therefore, the electrochemicalcell of the present invention has an electron movement path that isshorter than that of the known electrochemical cell, and accordingly,the current density-voltage characteristics of the electrochemical cellmay be excellent due to the short electron movement path, therebyreducing energy consumption during electrolysis.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

What is claimed is:
 1. A membrane electrode assembly for anelectrochemical cell comprising: a polymer electrolyte membrane; a firstelectrochemical reaction layer formed on one side of the polymerelectrolyte membrane to allow an oxidation reaction to occur thereon; afirst electron-conductive layer formed between the polymer electrolytemembrane and the first electrochemical reaction layer; a secondelectrochemical reaction layer formed on a remaining side of the polymerelectrolyte membrane to allow a reduction reaction to occur thereon; anda second electron-conductive layer formed between the polymerelectrolyte membrane and the second electrochemical reaction layer,wherein the first electron-conductive layer and the secondelectron-conductive layer include a porous metal.
 2. The membraneelectrode assembly of claim 1, wherein the porous metal further includesa coating layer including a platinum group applied on a surface thereof.3. The membrane electrode assembly of claim 1, wherein the firstelectron-conductive layer and the second electron-conductive layer eachhave a thickness of 0.1 to 1 mm.
 4. The membrane electrode assembly ofclaim 1, wherein the first electron-conductive layer and the secondelectron-conductive layer include any one among platinum, palladium,rhodium, iridium, ruthenium, osmium, carbon, gold, tantalum, tin,indium, nickel, tungsten, manganese, and titanium, or a complexincluding two or more thereof.
 5. The membrane electrode assembly ofclaim 1, wherein the first electrochemical reaction layer and the secondelectrochemical reaction layer each includes an electrochemical catalyst(or the electrochemical catalyst on a carrier), an ionic conductor, anda binder.
 6. The membrane electrode assembly of claim 5, wherein theelectrochemical catalyst includes any one among platinum group elementsincluding platinum, palladium, ruthenium, iridium, rhodium, and osmium,any one metal among iron, lead, copper, chromium, cobalt, nickel,manganese, vanadium, molybdenum, gallium, and aluminum, or alloys,oxides, or double oxides thereof.
 7. The membrane electrode assembly ofclaim 1, wherein elements constituting the membrane electrode assemblyinclude the polymer electrolyte membrane, the first electron-conductivelayer, the second electron-conductive layer, the first electrochemicalreaction layer, and the second electrochemical reaction layer indescending order of size.